Effect Of Silicon Content On Microstructure And Properties Of Heat Resistant Ductile Iron

Silicon-based heat-resistant cast iron accounts for more than 80%of the total production of heat-resistant cast iron.Silicon-based heat-resistant gray cast iron is widely used at present,.It has a wide range of alloy elements,low production cost,good mechanical properties and high temperature performance,and is widely used.With the development of transportation technology and ultra-supercritical power generation technology,at the same time,the use conditions of automobile engine parts and generator unit parts are becoming more and more stringent,and the mechanical properties and heat resistance temperature are getting higher.The strength and high temperature resistance of silicon-based heat-resistant gray iron have been difficult to continue to meet the market demand.It is urgent to develop a material with higher strength and high temperature resistance.Compared with flake graphite in gray iron,spherical graphite in ductile iron has less cleavage effect on the matrix,so it has better mechanical properties.Studies have shown that adding a large amount of silicon into cast iron can improve the high temperature resistance,but there are few studies on the mechanical properties and high temperature properties of silicon-based heat-resistant ductile iron.Therefore,this paper further improves the silicon ductile iron.On the premise of ensuring the original comprehensive mechanical properties,the high-temperature high temperature high temperature oxidation resistance and thermal fatigue resistance are further improved by adding alloy elements and optimizing the silicon content,and a higher performance heat-resistant ductile iron is developed.In this experiment,Y-type standard blocks with 2.8 wt.%,3.3 wt.%,3.8 wt.%,4.3 wt.%and 4.8 wt.%silicon content were cast to study the effect of silicon content on the as-cast microstructure and properties of ductile iron.The results show that with the increase of silicon content,the roundness and spheroidization rate of spherical graphite decrease gradually,and the ferrite content in the matrix increases from48.94%to 91.35%.With the increase of silicon content,the hardness of as-cast ductile iron increases from 276 HBW to 345 HBW,and the tensile strength at room temperature increases first and then decreases,while the elongation decreases.When the silicon content is 3.8 wt.%,the tensile strength reaches the maximum of 726 MPa and the elongation is 1.9%.The high temperature tensile strength increased with the increase of silicon content,from 437 MPa to 531 MPa,and the elongation decreased from 11.3%to 6.1%.After high temperature graphitization annealing,the roundness of spheroidal graphite is improved,and the matrix microstructure is mainly ferrite.With the increase of silicon content,the hardness of heat-treated ductile iron increases from 232 HBW to 329 HBW.The tensile strength at room temperature increases first and then decreases,and the elongation decreases.When the silicon content is 3.3wt.%,the tensile strength reaches the highest value of 624 MPa and the elongation is7.1%.The tensile strength at high temperature increased from 342 MPa to 532 MPa,and the elongation decreased from 20%to 8.1%.The as cast and heat treated samples are placed at 700℃and 800℃respectively for high-temperature oxidation resistance test.The spherical graphite on the surface of the sample reacts with oxygen and disappears.Fe₂Si O₄ and Cr₂O₃ oxide films are formed on the surface of the sample,which will hinder the subsequent oxidation process and improve the high-temperature oxidation resistance of the alloy.With the increase of silicon content,more silicon elements are enriched on the surface and react with oxygen atoms to form a thicker Fe₂Si O₄ oxide film,which further improves the oxidation resistance of the alloy.Under the same temperature and silicon content,there is no significant difference in the thickness and quality of oxide film between as cast and heat treated samples.It is judged that the matrix structure has little effect on the high-temperature oxidation resistance of the samples.The as-cast and heat-treated samples with five components were placed at 700℃and 800℃respectively for thermal fatigue performance test.The crack initiation was at the stress concentration position at the V-notch,and the crack propagation rates at700℃and 800℃showed‘S’curve.At the same temperature,with the increase of silicon content,the crack length also shows a trend of gradually longer.The thermal fatigue resistance of the ferrite matrix sample after heat treatment is stronger than that of the as-cast pearlite matrix sample.